Methods for capturing CO2 from a CO2-containing gas stream are known in the art. The most commonly applied method is post combustion CO2 capture, which is generally carried out in the following way: a CO2 containing gas stream coming from, e.g., a coal fired power plant is contacted in an absorption column with an amine-containing aqueous liquid at a temperature of about 40° C., which results in absorption of the CO2. A commonly used amine is monoethanolamine (MEA). The CO2-containing liquid stream is subsequently brought to a desorption column, which is operated at elevated temperature (>100° C.). In this column the CO2 is desorbed, after which it is compressed and ready for further use. The depleted liquid is returned to the absorption column. A weakness of this process is the relatively high energy consumption of the process. This high energy consumption is a result of the relatively high temperature needed in the desorption column to obtain sufficient CO2 desorption.
There have been many investigations into CO2 capture processes with lower energy consumption. One particularly interesting idea concerns the use of a phase separation to enhance the desorption of CO2 in such a way that it occurs at lower temperatures. For example, US2010/0288126 describes a process wherein a CO2-containing gas stream is contacted with a CO2-absorbing agent, followed by at least partial removal of the CO2 absorbing agent and inducing a phase separation accompanied by the release of CO2. US2010/0104490 describes an integrated process using such phase separation steps.
JP2010-155753 describes a process for capturing CO2 wherein CO2 is absorbed using an aqueous solution comprising a nitrogen-containing compound and a high molecular weight temperature-sensitive compound. The nitrogen-containing compound and the high molecular weight temperature-sensitive compound are either in separate molecules, or in the form of a block-copolymer.
Hoshino et al. (Y. Hoshino, Reversible Absorption of CO2 triggered by Phase Transition of Amine-containing Micro- and Nano-gel Particles, Journal of the American Chemical Society 2012, 134, 18177-18180) describes a CO2-catching polymer which comprises an amine-containing monomer built into a non-CO2 responsive thermosensitive polymer. The polymer also contains crosslinker, and is a hydrogel. At low temperatures the hydrogel is hydrophilic in nature but cannot dissolve because of the crosslinker; it acts like a gel-like material. At high temperatures the hydrogel is hydrophobic in nature and is insoluble; it acts like a solid phase.
WO2013/27668 describes the use of a polymer comprising an amine-containing monomer built into a non-CO2 responsive thermosensitive polymer in a system to generate an ion concentration, or to adsorb CO2.
While the systems described in these references are attractive in principle since they enable CO2 desorption at a lower temperature than the MEA system, they suffer from various disadvantages. In particular, it has been found that the net CO2 sorption of the process may be improved.
The quantity of CO2 absorbed can be expressed in the loading α, which is defined as follows:
  α  =            moles      ⁢                          ⁢              CO        2            ⁢                          ⁢      absorbed              moles      ⁢                          ⁢      amines      ⁢                          ⁢      present      
The net CO2 sorption, αnet, in the process can be defined as the difference in α at the end of the adsorption step (αT1) and at the end of the desorption step (αT2):αnet=αT1−αT2 
Further problems that may be recognized in the processes of the prior art are the following. The amine compounds used in US2010/0288126, and to a lesser degree those of JP2010-155753 are relatively volatile and susceptible to thermal degradation. Due to the temperature swing applied, this is a major issue in the commercial operation of the processes described therein. The system of Hoshino et al. suffers from slow absorption kinetics due to the use of a hydrogel.
The present invention provides a solution for these problems.